On the day he would put on the pinstripes for the first time, at his introductory news conference at Yankee Stadium, pitcher Masahiro Tanaka arrived at the airport in a Boeing 787 Dreamliner, which he had chartered from Tokyo for $195,000. It could accommodate 200 passengers, but on that day held only six: Tanaka, his pop-star wife, a trio of representatives and a brown poodle named Haru. For New York Yankees fans, the only appropriate response was a roll of the eyes and a pointed question: Is this guy too soft to cut it here?

The answer, four games into Tanaka’s major league career, arrived in the fourth inning Tuesday night at Fenway Park. The Boston Red Sox had just cut a four-run lead in half with back-to-back, one-out homers, the first a 482-foot moon-shot by David Ortiz that featured a pimp-job pose-and-stroll by the burly slugger, the second a screaming missile by Mike Napoli that was still rising when it cleared the Green Monster.

This was where it always happens: The Red Sox smell blood. The crowd roars to life. The walls of the cozy, storied bandbox start to close in. The rookie pitcher melts down and, a few batters later, walks off the mound with both the lead and his dignity gone.

Only that is not what happened. Tanaka, 25, calmly stepped back onto the mound, closed out the inning by freezing Xander Bogaerts, the Red Sox’ highly touted rookie shortstop, with a 93-mph fastball, and cruised through the rest of the night without incident.

“I tried to tell myself, ‘I gave up those runs,’ ” Tanaka said through his interpreter, “ ‘but no more.’ ”

With 71/3 poised, efficient innings in a 9-3 victory, Tanaka passed the biggest test of his young big league career, improved his record to 3-0 with a 2.15 ERA and at least temporarily put to rest any questions about his fortitude. He had stared down one of the most harrowing settings in baseball for any pitcher — on the ropes, at Fenway Park, against the Red Sox’ pitch-eating offense — and come out on the other side unscathed.

“He has a great presence on the mound. It doesn't seem like he's fazed by too much,” Yankees shortstop Derek Jeter said. “He has a lot of confidence in his ability, on top of having a lot of ability.”

The Yankees are slowly realizing what they have on their hands in Tanaka, and it gets better with every trip to the mound. They thought enough of his sheer ability to give him a $155 million contract this winter — on top of the $20 million posting fee they had to pay to his Japanese team, the Rakuten Golden Eagles, in order to negotiate a major league contract — but even then, General Manager Brian Cashman referred to him during spring training as a “potential No. 3” starter.

Perhaps it was smart to dampen expectations. In Boston, they still speak Daisuke Matsuzaka’s name with a mixture of regret and frustration. The last Japanese pitcher to enter the Yankees-Red Sox rivalry with this much hype, Matsuzaka — who cost Boston $103.1 million in contract and posting fees in 2006 — had two good years in Boston, but eventually plummeted, thanks to injuries and a maddening inability to throw strikes, all the way to the minor leagues.

But scouting and statistical analysis have improved since then, and less than a month into his rookie season, it is clear Tanaka — bigger, stronger and with better pure stuff — is no Matsuzaka. Nor does he appear to be a No. 3 starter. In fact, if Tanaka isn’t the best pitcher in the American League thus far, he is, at very least, the most uncomfortable at-bat for opposing hitters.

With a fastball that reaches 95 mph, a deceitful curveball, a slider good enough to be most pitchers’ “out” pitch and two distinct split-finger pitches — a tumbling one that he throws for strikes early in the count, and a devastating one that he throws with two strikes — Tanaka induces some of the ugliest swings in the game, and judging from Tuesday night’s start at Fenway, some of the most curious sideways glances.

And because Tanaka is such a strike-thrower, the Red Sox couldn’t wear him down the way they normally do to opposing starters, taking pitches, grinding out long at-bats, running up the pitch count. While his Red Sox counterpart, lefty Jon Lester, needed 118 pitches to plod through 42/3 tedious innings, Tanaka recorded eight additional outs while throwing 13 fewer pitches.

“He's not just up there throwing,” Jeter said. “He has a plan and he has been executing.”

Through four starts, Tanaka has struck out 35 batters in 291/3 innings while walking only two — a staggering ratio of strikeouts to walks (17.5) that, were it to hold up all season, would obliterate Bret Saberhagen’s all-time record (11.0) set in 1994. If Tanaka gets two strikes on you, you might as well give up. Opposing hitters have batted just .081 with a .222 on-base-plus-slugging percentage with two strikes against him this season, compared with a league average of .179 and .521.

“He looks like the real deal to me,” said David Cone, a former Cy Young-winning pitcher who is an analyst for the Yankees’ television network. “The quality of his pitches is definitely a notch above. And for being 25 years old, he’s incredibly polished.”

By this point, it would be understandable if Tanaka had completely forgotten what it feels like to lose. He hasn’t taken an ‘L’ in a regular season game since Aug. 19, 2012, in the Japanese league. He finished that season with four straight wins for Rakuten, then went 24-0 in 2013. Tack on his three wins for the Yankees this year and that’s a 31-0 mark since his last regular season loss. (He did lose a postseason game for Rakuten, in Game 6 of the Japan Series against Yomiuri last October, a game in which he threw 160 pitches. But he came back the next night to earn the save in Game 7.)

A key part of the Yankees’ blueprint with Tanaka — once they outbid the Diamondbacks and Cubs, among others — was to hide him from the Red Sox. He didn’t face them in spring training, and the Yankees tweaked their rotation so he missed the four-game series the teams played in the Bronx earlier this month. Finally, on Tuesday night, it was time to turn him loose at Fenway, if only to answer the pressing question.

“I think we’ll continue to learn about him all year long,” Yankees Manager Joe Girardi said. “There [are] obviously things we haven't seen him go through yet that he’s probably going to go through. We seem to learn something about him every night.”

The Yankees indeed learned something about Tanaka on Tuesday night — as did, for that matter, the Red Sox. The Yankees learned they just might have an ace on their hands. The Red Sox learned they have a new problem on theirs.

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